Login to Cacti admin site and click "Plugin Management" on the left menu and click cursor on the Settings field icon in the right pane.

[3]

Click cursor button to enable the plugin.

[4]

After enabling, the stauts turns to Active.

[5]

Click "Settings" on the left menu and move to "Mail/DNS" tab on the right pane and input items like follows and click "Save" button.
Test Mail ⇒ recipient's address for test mail
Mail Services ⇒ the service for using sending email
From Email Address ⇒ the address for "From"
From Name ⇒ the name for "From"

After saving settings, click "Send a Test Email" to make sure email is sent normally.